The present invention relates generally to a system and method for magnetic resonance (MR) imaging, and more particularly, to an MR system and pulse sequence which slice-selectively excites multiple frequencies for quick and efficient imaging. Spectral-spatial radio frequency (RF) pulses may be used to create magnetization in specific frequency profiles without significantly affecting neighboring slices or nearby frequency ranges. The signals resulting from such pulses are then read in an order reversed from the order in which the pulses were applied.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which has a frequency near the Larmor frequency, the net aligned moment, or “longitudinal magnetization”, MZ, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins after the excitation signal B1 is terminated and this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (Gx, Gy, and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The set of received nuclear magnetic resonance (NMR) signals resulting from a scan sequence are digitized and sent to a data processing unit for image reconstruction using one of many well known reconstruction techniques. It is desirable that the imaging process, from data acquisition to reconstruction, be performed as quickly as possible for improved patient comfort and throughput.
For some procedures and investigations, it is also desirable for MR images to display spectral information in addition to spatial information. The traditional method for creating such images is known as “chemical shift imaging” (CSI). CSI has been employed to monitor metabolic and other internal processes of patients, including imaging hyperpolarized substances such as 13-C labeled contrast agents and metabolites thereof. The hyperpolarization of contrast agents tends to have a very limited lifetime; typical T1 lifetimes are on the order of a few minutes in vivo.
However, CSI, as a sequence for imaging hyperpolarized substances, has some drawbacks which limit available signal-to-noise ratio and thus image quality. For example, CSI tends to acquire data slowly, considering the short lifetimes of the increased magnetization of hyperpolarized substances. In addition, CSI typically uses a large number of RF excitations. Each excitation irretrievably destroys the magnetization of hyperpolarized substances. Additionally, MR procedures which require very fast or periodic data acquisition (such as cardiac imaging, or metabolic imaging of the heart) are difficult to perform with CSI sequences. CSI typically takes about 15 seconds to complete, whereas cardiac and related metabolic imaging should be completed within a few heartbeats or a few seconds.
Non-CSI techniques for imaging hyperpolarized substances without acquiring spectral information include single-shot techniques (e.g. a strong RF pulse which destroys all magnetization and attempts to acquire all data for multiple metabolites at once) or imaging with a large number of small flip-angle RF pulses (e.g. multiple excitations with flip angles on the order of 1 or 2 degrees). These approaches excite all frequency ranges for metabolites of interest simultaneously, destroying hyperpolarization of all metabolites with each pulse. In addition, when pulses of lower flip angle are used, a lower signal-to-noise ratio (SNR), and hence a lower image resolution, is the result.
It would therefore be desirable to have a system and method which overcomes the aforementioned drawbacks of MR imaging with spectral information and hyperpolarization. Specifically, it would be desirable to excite and image hyperpolarized agents and metabolites thereof within a short time, while efficiently utilizing the full magnetization of each substance and acquiring spectral information.